The invention relates to an assembly for inspecting a surface of a sample.
One of the routine steps in the production process of integrated circuits is the inspection of patterned surfaces, especially when starting up a new design. A substantial part of the whole 300 mm wafer is imaged to check for defects in the pattern and for particles imbedded in the pattern or on top of the wafer. This kind of inspection is presently performed by high-throughput optical microscopy in dedicated instruments.
With the progress in lithography, the instruments must detect smaller and smaller defects and particles. A problem is that the light scattering from particles rapidly decreases when the particle's size decreases, so the signal-to-background (and noise) ratio is decreasing.
In order to solve this problem, electron beam inspection machines have been used and for some purposes are still in use. Electron beam inspection machines can have a much higher resolution than optical system. However, electron beam inspection machines are limited in the speed at which they can inspect a wafer. In order to increase the speed, multi-column electron beam systems have been proposed.
WO 2004/017355, for example, discloses an example of an electron optics assembly for a multi-column electron beam inspection tool having about 52 electron beam columns which are distributed over an area of the semiconductor wafer. Each column comprises its own electron gun. According to WO 2004/017355 it is advantageous that the assembly comprises one or more electron optical components which are single structures for the whole assembly of electron beam columns, such as the first accelerator electrode, the final accelerator electrode, the focus electrode mounting plate and the gun mounting plate. These single structures provide mechanical integrity to the electron optics assembly and facilitates the manufacturing of the assembly.
Due to the use of 52 electron beam columns, productivity can be increased. However, for an electron beam inspection apparatus having a throughput in the order of one wafer per hour, for example, this number of electron beam columns is too little, as illustrated below:
To make an image with a reasonable signal-to-noise ratio, in the order of 300 to 400 primary electrons per pixel are needed (accounting for a quantum detection efficiency of about 0.3). A semiconductor wafer with a diameter of 300 mm contains approximately 7×1014 pixels of 10×10 nm for detecting a 10 nm defect. For obtaining a throughput of one wafer per hour, a current of approximately 10 μA is required. The actual required current depends on many factors such as the defect contrast, choice of beam size and required defect capture rate. However, the required current will be in this order of magnitude.
The typical currents in electron microscopes with high brightness sources are in the order of nA's. Thus, the desired throughput of 1 wafer per hour can only be obtained using for example 10,000 electron beam columns or more in parallel. Such a system requires that the electron beam columns are miniaturized to a column footprint of approximately 7 mm2, which is difficult and costly to manufacture.
It is an object of the present invention to provide an alternative inspection apparatus, which allows for high throughput inspection of samples, in particular semiconductor wafers.